Coaxial tubing is commonly used to provide flow for two fluids (i.e., gases or fluids) and typically comprises an inner tube supported between the walls of an outer tube. For example, such tubing is often utilized to carry toxic or hazardous gases used in connection with semiconductor manufacturing wherein the inner tube carries the toxic gas and an air space formed between the two parallel tubes carries an uninterrupted flow of purge gas. Should a leak occur in the inner tube, the released toxic or hazardous gas will mix with the purge gas in the annular space between the inner and outer tubes and thus be prevented from escaping into the ambient atmosphere. Alternatively, the purge gas is at a higher pressure than the toxic gas so that the purge gas blocks the release of the toxic gas from the inner tube. A leak is detected as a drop in the pressure of the purge gas. In addition, the outer tube serves to protect the inner tube from contact with objects which may accidentally breach the inner tube and allow the gas carried therein to leak into the surrounding air.
The tubing is typically provided in lengths which are cut to fit a particular installation. In addition, certain fittings are provided including elbows, tees, couplers, and the like, to interconnect the various lengths of tubing. Commonly, the coaxial tubing is connected by first welding the inner tubing to a fitting and then welding the outer tube to the fitting, using a sliding sleeve, for example To assemble the individual lengths into a unified structure, each weld must be performed separately. The process is thus very time consuming. Furthermore, the process is difficult to automate, particularly when it must be performed at an installation site for a unique configuration. This is a particular disadvantage since the installation sites are typically clean rooms, or the like, where the residue from cutting and welding is undesirable.
Couplings have been developed which can be partially assembled beforehand, leaving relatively simple connections to be made at the installation site. One such coupling is disclosed in U.S. Pat. No. 4,732,414 entitled "Joint for Coaxial Pipe" wherein the coupling comprises a male and female joint portion. The end surfaces of the joint portions have opposed protrusions which engage a common metallic packing. However, such a design is disadvantageous as the common packing provides only a single sealing surface between the inner and outer tubes in the pipe and, thus, accurate alignment of the packing is difficult to obtain. In addition, this design requires precise parallel formation of the protrusions on the male and female joint portions for both the inner seal and the outer seal in order to engage the packing, resulting in increased manufacturing time and expense.
Soviet Union Patent 461,269 discloses a twin bore pipe coupling including a nipple welded to one pipe and a union welded to the other in a coaxial pipe. When a nut is screwed onto the union, a sealing insert is trapped and the sealing of the external and internal bore is complete. This type of coupling is disadvantageous because the rotational forces applied to screw the nut onto the union exert similar rotational forces on the sealing insert which could damage the insert. The coupling is not applicable to high pressure applications because the seal created between the external and internal bore is not leak-tight Furthermore, the coupling is not a zero-clearance fitting, so that if the coupling breaks or is damaged, the entire pipe section must be disconnected and disassembled.
UK Patent Application 2,034,839 entitled "A coupling element and pipe coupling for use with jacketed pipes" also discloses a coupling for use with coaxial tubing. The coupling includes first and second coupling elements which are drawn together using an internally threaded connecting nut. Two elastomeric sealing rings are compressed between the two coupling elements to ensure fluid-tight connections. The sealing rings cannot however, be used with corrosive gases because the gas will act to deteriorate the elastomeric seals resulting in leaks between the coupling elements.